1. Field of the Invention
The invention relates to a crucible induction furnace with at least two coils, which are connected in parallel to a load-commutated tuned circuit converter, run round the wall of a vertical melting crucible, are disposed one behind the other along the axis of the crucible and are connected in series with a capacitive impedance.
2. Description of The Prior Art
It is a universally known practice, when melting metals in a crucible induction furnace of the type stated above, to use induction coils to generate eddy currents in the material to be melted and thereby heat the metal. The melting power transmitted by a coil increases with the magnitude and frequency of a voltage applied to the coil. In order to be able to operate with higher frequencies than the mains frequency, use is generally made of tuned circuit converters, to the secondary side of which the induction coils are connected. However, the level of the secondary voltage in the case of tuned circuit converters is limited due to the tolerance to voltage of the semiconductor components.
It is furthermore common practice to increase the voltage applied to the coils of a crucible induction furnace by connecting a capacitive impedance in series with mutually parallel induction coils to the secondary side of a tuned circuit converter.
The purpose of the upstream capacitive impedance is to bring the tuned circuit into the resonant oscillation range. To explain its action, an ideal series tuned circuit with a coil of inductance L, a capacitor of capacitance C and a vanishing resistance is considered below. In this case, the following equation applies to the resonant frequency f.sub.0 : ##EQU1## If a tuned circuit of this kind is operated with an operating frequency f and an operating voltage U.sub.1, the voltage U.sub.2 in accordance with the equation: EQU U.sub.2 =U.sub.1 /(1-(f/f.sub.0).sup.2) (II)
can be picked off between the input and the output of the coil.
In the circuit described above belonging to a crucible induction furnace and corresponding to the prior art, the same voltage is applied to all the coils. Such a circuit arrangement makes sense when the power requirement is the same at all the coils.